The present invention relates to an objective lens for an optical pick-up used for an optical disc drive, which is capable of recording data to and/or reproducing data from a plurality of types of optical discs having different recording densities and having different thicknesses of cover layers.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
When the recording/reproducing for the DVD having a higher recording density is performed, a smaller beam spot diameter is required on the data recording layer of the DVD relative to a beam spot used for the CD having a lower recording density. For this reason, the optical pick-up is configured such that a NA (numerical aperture) is changed to a higher value to obtain a smaller beam spot diameter when the DVD is used and that the NA is changed to a lower value to obtain a larger beam spot diameter when the CD or CD-R is used.
The diameter of the beam spot decreases as a wavelength of the laser beam decreases. Therefore, in general, a laser beam having a wavelength of about 660 nm, which is lower than a wavelength of about 780 nm for the CD, is used for the DVD.
A condition of a spherical aberration in an optical system of the optical pick-up changes depending on a thickness of the cover layer of the optical disc being used. Therefore, it is required to correct the spherical aberration caused when the optical disc is changed to another one having different thickness of cover layer.
Japanese Patent Provisional Publication No. 2000-81566 discloses a CD/DVD compatible objective lens (i.e., an objective lens of an optical pick-up that can be used for both of the DVD and CD). The CD/DVD compatible objective lens has a diffracting structure on one of its lens surfaces. The diffracting structure has a plurality of concentric ring-shaped steps having a function of properly converging an incident beam on a data recording layer of an optical disc regardless of the thickness of the cover layer of the optical disc being used.
That is, the diffracting structure has wavelength dependence such that the spherical aberration changes when the wavelength of the incident beam changes. Consequently, the CD/DVD compatible objective lens is capable of supporting various types of optical discs having different thicknesses of the cover layers.
More specifically, the lens surface on which the diffracting structure is formed has an inner area which provides an NA required for the optical disc having relatively low recording density (e.g., CD), and an outer area which is situated outside the inner area and which provides an NA required for the optical disc having relatively high recording density (e.g., DVD).
The diffracting structure within the inner area has a function that the laser beam for the CD is properly converged on the data recording layer of the CD and that the laser beam for the DVD is properly converged on the data recording layer of the DVD. The diffracting structure within the outer area has a function that the laser beam for the CD does not contribute to the formation of the beam spot for the CD and the laser beam for the DVD is most properly converged on the data recording layer of the DVD.
With the above mentioned structure, with regard to the laser beam for CD, only the laser beam passed through the inner area is properly converged on the data recording layer of the CD, while the laser beam passed through the outer area diffuses on the data recording layer of the CD. Consequently, the beam spot having relatively large diameter is formed on the data recording layer of the CD.
With regard to the beam for DVD, both of the inner and outer areas converge the laser beam passing therethrough on the data recording layer of the DVD. Consequently, the beam spot having relatively small diameter is formed on the data recording layer of the DVD.
Recently, new technical standards concerning optical discs having further higher recording densities are proposed. One of such optical discs is an HD DVD (High Definition DVD) having a recording density higher than that of the DVD. The HD DVD has a thickness of the cover layer smaller than or equal to that of the DVD. For the HD DVD, the laser beam having a wavelength smaller than that of the DVD (a so-called blue laser having a wavelength of, for example, about 405 nm) is required.
Against the background of the start of the practical use of the optical discs having further higher recording densities such as an HD DVD, the demand for optical disc devices which can provide compatibility among existing optical discs (e.g., CD and DVD) and the optical discs of the new technical standards (e.g., HD DVD) is increasing. To accomplish such an optical disc device, an objective lens that is capable of converging incident beam on the data recording layers of various types of optical discs including the CD, DVD and HD DVD is required.
However, the objective lens disclosed in the publication No. 2000-81566 is not configured to support the HD DVD. That is, when the blue laser beam is incident on the objective lens disclosed in the publication, aberrations including a spherical aberration are caused and therefore a beam spot suitable for the recordation and the reproduction of data of the HD DVD can not be formed.
Japanese Patent Provisional Publication No. 2001-195769 discloses an objective lens and an optical pick-up configured to support the HD DVD, DVD and CD. The optical pick-up disclosed in the publication 2001-195769 is configured such that a collimated light beam is incident on the objective lens when the HD DVD and the DVD is used, and a diverging beam is incident on the objective lens when the CD is used. However, in this publication no explanation is made on what type of a diffracting structure is desirable to form a beam spot suitable for recording data to and/or reproducing data from each of the HD DVD, DVD and CD.
In many cases, a diffracting structure is configured to have a sawtooth shape (i.e., a blazed shape) in order to increase the amount of light for a particular order of diffracted light. Japanese Patent Provisional Publication No. 2000-75118 discloses an objective lens having a diffracting structure. The diffracting structure disclosed in the publication 2000-75118 is configured to effectively obtain a certain order of diffracted light within a wide range of wavelength. The diffracting structure has a laminated structure of two diffraction gratings.
However, if the two diffraction gratings are laminated with respect to each other, there may be a case where the two diffraction gratings are laterally shifted with respect to each other and thereby the amount of light decreases. For this reason, the diffracting structure disclosed in 2000-75118 requires a manufacturing process with a high degree of precision. Since the high degree of manufacturing precision is required, it is not desirable to use the objective lens disclosed in 2000-75118 for an objective lens, used for an optical pick-up, which has a very low cost and which is generally mass-produced.